Reasonable suspicion or reasonable articulable suspicion is a legal standard of proof that in United States law is less than probable cause, the legal standard for arrests and warrants, but more than an "inchoate and unparticularized suspicion or 'hunch'"; it must be based on "specific and articulable facts", "taken together with rational inferences from those facts", and the suspicion must be associated with the specific individual. If police additionally have reasonable suspicion that a person so detained is armed and dangerous, they may "frisk" the person for weapons, but not for contraband like drugs. However, if the police develop probable cause during a weapons frisk (by feeling something that could be a weapon or contraband, for example), they may then conduct a full search. Reasonable suspicion is evaluated using the "reasonable person" or "reasonable officer" standard, in which said person in the same circumstances could reasonably suspect a person has been, is, or is about to be engaged in criminal activity; it depends upon the totality of circumstances, and can result from a combination of particular facts, even if each is individually innocuous.

In Terry v. Ohio, the U.S. Supreme Court ruled that a person can be stopped and briefly detained by a police officer based on a reasonable suspicion of involvement in a punishable crime. If the officer has reasonable suspicion the detainee is armed, the officer may perform a "pat-down" of the person's outer garments for weapons. Such a detention does not violate the Fourth Amendment prohibition on unreasonable searches and seizure, though it must be brief. Reasonable suspicion does not provide grounds for arrest; however, an arrest can be made if facts discovered during the detention provide probable cause that the suspect has committed a crime. (Note: Probable cause cannot be after the fact. Any added probable cause after the fact would be inadmissible in a court of law.)

In Hiibel v. Sixth Judicial District Court of Nevada the Court further established that a state may require, by law, that a person verbally identify himself or herself to an officer during a stop; some states (e.g., Colorado) require that a person detained provide additional information.

However, as of 2020, only 24 states have "stop and identify" statutes on the books.

A brief, non-custodial traffic stop is considered a "seizure" for the purposes of the 4th Amendment and must therefore be supported by reasonable suspicion or probable cause. The investigating officer must weigh the totality of the circumstances to determine whether sufficient objective facts exist to elicit reasonable suspicion that the driver is engaged in criminal activity. If the investigating officer witnesses the driver commit a traffic violation (even if they are mistaken about what constitutes a violation, see Heien v. North Carolina), then said violation generally constitutes probable cause for the officer to stop the vehicle. The officer may detain the driver and any passengers of the vehicle for long enough to confirm and/or deny their suspicions.

If the suspicions of the officer are confirmed, then there may be probable cause to either search the vehicle and/or arrest its occupant(s), depending on the nature of the suspected violations. In Illinois v. Caballes, the Supreme Court held that a drug dog may sniff the exterior of a vehicle during a traffic stop so long as any delay in calling the dog to the scene does not unreasonably prolong the traffic stop. The use of a drug dog is sui generis and a dog's sniff is not considered a search in and of itself under the 4th Amendment.

Police may also set up roadblocks and stop drivers without particularized reasonable suspicion that the stopped individual is engaged in criminal activity, so long as the plan for the stop is applied neutrally, for instance, driving while intoxicated – so long as all vehicles are stopped or every third vehicle is stopped, or some other reasonably neutral policy is applied for stopping vehicles. Note that some states also impose additional notice requirements for roadblocks, such as appropriate signage and/or flashing blue or red lights.

New Jersey has set the precedent that probable cause is not necessary to search a student on school grounds; reasonable suspicion is enough to search a student's belongings. Overly intrusive searches, like a body cavity search, require probable cause.